In the fundamental research larger amounts of cell material for in vitro experiments are needed, for example to execute functional assays or to analyze the adhesion and proliferation behavior, the gene and protein expression. Also in application-oriented disciplines of life sciences there is a need of cell material, for example for the so-called tissue engineering or the cell therapy.
Since cell donors cannot be strained permanently with tissue removal, the current availability of cell material, particularly of human origin, is unsatisfactory.
Primary cells, i.e. those cells that were isolated from an immediately previously still intact organ or tissue, have the disadvantage that their ability to divide is limited. In higher passages often the so-called cell senescence occurs. This applies especially to human primary cells. In this context it is spoken of the so-called Hayflick limit which refers to the limited number of cell divisions in eukaryotes, which a cell may undergo. Upon reaching this limit the programmed cell death is initiated because the telomeres reach a critically reduced length.
A certain advantage have stem cells which initially comprise strong proliferation ability, however, their availability is also limited by the occurrence of cell senescence in higher passages. In addition, stem cells, especially those of mesenchymal character, increasingly lose their differentiation ability in higher passages and are therefore only suitable to a limited extent for the purpose mentioned at the outset and especially for the examination of the differentiation processes.
In order to overcome these limits, immortalized cell lines from different tissue types were generated, “Immortalization” shall be understood as the immortal-making of cells. The limitation of the number of cell divisions by the Hayflick limit is abrogated thereby. Immortalized cells, or cell lines, respectively, may divide any number of times in contrast to normal cells and can proliferate in cell culture in an unlimited manner. An immortalization may be reached, for example, by an infection of the cells with particular viruses, i.e. by viral transduction or by fusion with tumor cells. Immortalized cells often retain the self-function of primary cells from which they were generated.